There are four files of correspondence between Clark and Blackwell. The letters are not entered individually. Their correspondence began because Clark was writing a biography of Albert Einstein. Clark began his work on his biography of Russell in late 1971.

1260a, Oct. 1968-March 1974. Contains transcriptions of some of BR's letters to Ottoline Morrell. There are also third-party letters in this file including some by William Ready and Anton Felton.

1260b, Apr.-Dec. 1974. Also contains John G. Slater letters.

1260c, Jan.-Sept. 1975. There is also a copy of the dust jacket of The Life of Bertrand Russell as well as a printed pamphlet advertising the book, using an image of a different dust jacket cover (not used).

1260d, Oct. 1975-Oct. 1986. Concerns Clark's picture book, Bertrand Russell and His World and as well as the biography.

The files include third-party letters: Penguin Books, John G. Slater.